Microfluidics is the field of microscale fluid flow and control, typically through microscale fluid control features constructed on a substrate such as a glass chip. These devices can be used to manipulate liquid chemical and biological samples in order to conduct analysis, perform synthetic reactions, and the like.
A motive force often used in microfluidics is a phenomenon known as electroosmotic flow (EOF). EOF depends strongly on various aspects of charge mobility in these systems, and in particular, depends on having a high density of ionic groups of the same polarity on the walls or flow surfaces of a microfluidics component. For example, a typical microfluidics feature is a microchannel etched in a glass substrate. The interior surface of the channel is treated to expose free silanol groups, which can then be deprotonated to provide siloxide anions. A voltage is applied across the flow direction of the channel causing solvated counterions of the charged groups on the channel walls to move. Because the dimensions of a microfluidic channel are small, the layer of counterions at the flow surface contact a significant portion of the fluid in the channel. Thus, when the counterion layer moves, the entire volume of the channel moves nearly simultaneously, a mechanism known as “plug flow”. For many applications, plug flow is advantageous because it means that the components of a particular volume portion of the flow travel together, and are not spread out as they would be in a conventional pressure driven flow. Thus, on a microfluidics chip, precise volumes can be delivered from one location to another with a high degree of control.
Many features of a macroscale laboratory can thus be miniaturized using microfluidics, allowing significant reductions in the amount of costly, rare, or hazardous materials that are used. For example, microfluidics has the potential to make efficient use of biomolecules such as enzymes or catalytic antibodies, which are typically expensive or difficult to prepare in large quantities. It is particularly desirable that these molecules be reusable or recoverable, for example, by immobilizing them to a solid support, in order to further limit the quantities that are required.
A significant problem that must be solved, however, is the difficulty of immobilizing biomolecules with high biological activity while simultaneously maintaining acceptable EOF capability.
For example, enzymes have been attached to glass chips by covalent attachment to free silanol groups on the glass surface. However, the high pH necessary to provide the charged siloxide anions significantly decreases the stability and catalytic activity of enzymes. Another attempt functionalized the silanol groups with a linker group ending in an amine, which can then be covalently attached to the enzyme. However, this leads to a reduction in the number of available siloxide groups at the surface, and further, shields the groups that are present from the flow, resulting in poor EOF characteristics.
Other attempts have been made to provide polymers that specifically enhance EOF, for example, by using a charged polymer such as dextran sulfate. These polymers, however, are dynamically unstable coatings, i.e., are not substantially adhered, and so are eventually washed away by the flow. Furthermore, they are not easily functionalized with biocatalysts such as enzymes, and they do not maintain the catalytic activity of enzymes.
High enzyme activity can be maintained by encapsulating and/or covalently attaching enzymes to matrices, such as solgels, but such matrices typically fill the entire microfluidic channel, providing a severe impediment to fluid flow. Furthermore, many other examples of polymers exist to immobilize enzymes with high activity, but they are not designed to support EOF.
Therefore, there is a need to immobilize biomolecules on a microfluidics apparatus, while simultaneously maintaining high biological activity and high EOF capability.